/Engine/Source/Runtime/Engine/Public/GPUSkinVertexFactory.h
Get the vertex stream index for a vertex attribute. This will be -1 for attributes that haven't been set in AddVertexAttributes(). It may also be -1 for attributes that are read through manual vertex fetch.
int32 GetAttributeStreamIndex ( [EVertexAtttribute](API\Runtime\Engine\FGPUSkinPassthroughVertexFactory\EVertexAtttribute) InAttribute ) const